Knitted or woven fabrics, and stretched knitted or woven fabrics to which stretchability is imparted in particular, have been strongly desired in recent years in view of the wear comfort.
In order to satisfy such a desire, many knitted or woven fabrics to which stretchability is imparted by, for example, mingling a polyurethane-based fiber, have been developed.
However, the polyurethane-based fiber has the problems that because the fiber is hardly dyed with a dyestuffs employed for polyester, the dyeing process becomes complicated, and that the fiber is embrittled and the properties are deteriorated when used for a long period of time.
In order to avoid such drawbacks, application of the crimp yarn of a polyester-based fiber in place of a polyurethane-based fiber has been examined.
Many latent crimp fibers that are prepared by combining two types polymers in a side-by-side manner or eccentrically and that manifest crimp after heat treatment have been proposed. In particular, by utilizing the elongation recovery of a poly(trimethylene terephthalate) (hereinafter abbreviated to PTT), a latent crimp fiber has been prepared.
Prior literature on PTT-based latent crimp fibers includes, for example, Japanese Examined Patent Publication (Kokoku) No. 43-19108, Japanese Unexamined Patent Publication (Kokai) No. 2000-239927, Japanese Unexamined Patent Publication (Kokai) No. 2000-256918, Japanese Unexamined Patent Publication (Kokai) No. 2001-55634, Japanese Unexamined Patent Publication (Kokai) No. 2001-131837, European Patent (EP) No. 1059372, U.S. Pat. No. 6306499, Japanese Unexamined Patent Publication (Kokai) No. 2001-40537, Japanese Unexamined Patent Publication (Kokai) No. 2002-61031, Japanese Unexamined Patent Publication (Kokai) No. 2002-54029 and the like.
The prior literature discloses a side-by-side type two-component-based conjugate fiber and an eccentric sheath-core type conjugate fiber (both types being referred to as a PTT-based conjugate fiber) in which PTT is used for at least one component or two PTTs differing from each other in intrinsic viscosity are used for the two respective components. A soft feel and crimp manifestation properties are characteristic of the PTT-based conjugate fiber. Such prior art literature describes that the PTT-based conjugate fiber can be applied to various stretch knitted or woven fabrics or bulky knitted or woven fabrics by utilizing the excellent stretchability and elongation recovery of the fiber.
A PTT-based conjugate fiber is produced by a two-stage method wherein spinning and drawing are conducted in two stages, or a one-stage method wherein spinning and drawing are continuously conducted in one stage.
The one-stage method wherein spinning and drawing are continuously conducted is commonly called a direct spin-draw process, and is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2001-131837, Japanese Unexamined Patent Publication (Kokai) No. 2001-348734, Japanese Unexamined Patent Publication (Kokai) No. 2002-61031 and the like. The direct spin-draw process has the advantage that a PTT-based conjugate fiber can be produced at low cost in comparison with the two-stage method wherein spinning and drawing are conducted in two stages.
Production methods (direct spin-draw processes) of conjugate fibers for which PTT is not used are known in Japanese Unexamined Patent Publication (Kokai) No. 8-337916, Japanese Unexamined Patent Publication (Kokai) No. 9-87922, Japanese Unexamined Patent Publication (Kokai) No. 2001-288620 and the like. Such literature discloses methods of producing a highly crimpable conjugate fiber by stretching the fiber between the second and the third godet roll in the production of a poly(ethylene terephthalate)-based conjugate fiber (hereinafter poly(ethylene terephthatate) is referred to as PET).
However, a PET-based conjugate fiber obtained by a direct spin-draw process is not suited to blending with a natural fiber such as wool due to its low dye-affinity in comparison with a PTT-based conjugate fiber, and has the drawback that its applications are limited due to its significantly weak stretchability.
On the other hand, although the direct spin-draw process can produce a PTT-based conjugate fiber at low cost, it has become evident that the process has problems, as explained below, that are associated with the production and the fiber produced and that are caused by PTT.
[Problems During Production of PTT-Based Conjugate Fiber]
(I) Winding Stability
It is described in Japanese Unexamined Patent Publication (Kokai) No. 2001-131837 that the thermal shrinkage stress of the drawn yarn of a PTT-based conjugate fiber produced by a direct spin-draw process is preferably made high for the purpose of enhancing crimp manifestation. Moreover, it is described in the patent publication that, when the thermal shrinkage stress value of a PTT-based conjugate fiber is made 0.25 cN/dtex or more, the fiber has a crimp ratio of 10% or more even under a load of 3.5×10−3 cN/dtex. Specifically, in Example 11 of the patent publication, a PTT-based conjugate fiber having a thermal shrinkage stress of 0.30 cN/dtex is described. Moreover, it is also described that when the conjugate fiber is used for a woven fabric that has a hard twist or that has a large texture restraint force, the woven fabric manifests high crimpability.
However, production of a PTT-based conjugate fiber showing a thermal shrinkage stress value as high as 0.25 cN/dtex or more encounters difficulties in spinning and winding. In particular, when a PTT-based conjugate fiber showing a high thermal shrinkage stress is wound into a package by a direct spin-draw process, problems as explained below arise.
When the thermal shrinkage stress of a PTT-based conjugate fiber is increased in order to improve the crimpability, the elastic recovery of the fiber becomes high, which is a phenomenon specific to a PTT. As a result, the PTT-based conjugate fiber shrinks to produce a poor package form during winding, or to cause package tightening, so that the package can hardly be taken out of the winding machine. Furthermore, a PTT-based conjugate fiber having a high thermal shrinkage stress tends to show irregular winding (also termed a wound yarn edge drop) on the sides of the package during winding, and yarn breakage is likely to take place during unwinding the conjugate fiber from the package. Still furthermore, because the conjugate fiber is wound with a high winding tension, the problem that a lowering in the success ratio of automated change-over of the package occurs. Accordingly, industrial production of a PTT-based conjugate fiber showing a high thermal shrinkage stress value has heretofore been extremely difficult.
(II) Dyeing Quality
In order to solve such problems, regarding winding a PTT-based conjugate fiber, as mentioned above, Japanese Unexamined Patent Publication (Kokai) No. 2001-348734 discloses a method comprising providing a non-heating relaxation roll between a second hot roll and a winding machine, and relaxing the fiber. However, as a result of attempting to practice the method, the present inventors have found that the non-heating relaxation roll temperature is influenced by a heat transferred by the fiber heated by the second hot roll, and consequently the relaxation roll temperature rises to about 40 to 50° C.
Because the temperature agrees with the glass transition temperature of a PTT, it has become clear that a slight variation of the temperature greatly influences the winding tension and the quality of the PTT-based conjugate fiber. Because industrial production of the fiber with multi-spindles is essential, the above variation causes a variation in the dyeing level of the fiber among spindles. As a result, the problem that a lowering in the dyeing uniformity occurs.
[Problems During Post-Treatment]
(III) High Speed False Twisting Property
Although a PTT-based conjugate fiber obtained by a direct spin-draw process can be used for knitted or woven fabrics without further processing, a false-twisted yarn prepared therefrom can manifest high stretchability even in high density woven fabrics showing a high restraining force as fabrics (see WO 02/086211).
Even in false twisting a PTT-based conjugate fiber, a high processing speed is required in order to improve the productivity. When an attempt is made to false twist at high speed either a known PTT-based conjugate fiber, the PTT-based conjugate fiber disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2001-131837 and showing a high thermal shrinkage stress, or the bulky PTT-based conjugate fiber disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2002-61031, crimp manifested in the PTT-based conjugate fiber hinders false twisting, and contact resistance, to guides of the false twisting machine, increases. As a result, it has become evident that fluctuation of a false twisting tension causes yarn breakage or produces uneven dyeing in the false-twisted yarn.
(IV) Tail End Transfer
Because false twisting is continuously conducted, the package is usually changed over by tail end transfer. A PTT-based conjugate fiber showing a high thermal shrinkage stress such as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2001-131837 generally exhibits a rise (manifestation starting) of a thermal shrinkage stress at temperature as low as about 50° C.; therefore, the tail end transfer becomes very difficult. Specifically, the PTT-based conjugate fiber peeled off the package for yarn tying rapidly manifests crimp at room temperature, and a yarn—yarn knotting operation is hard to conduct. Moreover, it has become clear that because knotting is difficult, the yarn—yarn knot strength tends to become weak, and as a result, yarn breakage frequently occurs during tail end transfer.
Such problems arising during false twisting become serious ones that make the industrial production difficult when high speed false twisting is conducted at a speed of about 400 m/min or more.
(V) Stretchability
A false-twisted yarn is required to manifest not only bulkiness but also high stretchability. A false-twisted yarn of a conjugate fiber composed of a PET as one component and a copolymerized PET as the other component is described in prior literature “Manual of Technologies of Processing Filaments” (Edited by The Textile Machinery Society of Japan: p190, 1976). According to the prior literature, the stretchability of the false-twisted yarn obtained by false twisting a conjugate fiber of PET/copolymerized PET is merely equal to the stretchability of a false-twisted yarn which is made from only PET or only copolymerized PET. In fact, PET-based conjugate fibers, described in Japanese Unexamined Patent Publication (Kokai) No. 8-337916, Japanese Unexamined Patent Publication (Kokai) No. 9-87922 and Japanese Unexamined Patent Publication (Kokai) No. 2001-288620, show no improvement of stretchability even when subjected to false twisting.
It has recently been proposed in Japanese Unexamined Patent Publication (Kokai) No. 2002-327341 and Japanese Unexamined Patent Publication (Kokai) No. 2003-55846 to draw and false twist highly oriented undrawn yarns of PTT-based conjugate fibers. However, the present inventors have found after investigation that, because such a highly oriented undrawn yarn has a breaking elongation as high as from 100 to 250%, the thermal shrinkages between the two components become close to each other by drawing and false twisting in a high ratio, and a false-twisted yarn showing high stretchability and adaptable to high density woven fabrics (the false-twisted yarn being an object of the present invention) cannot be obtained.
Therefore, creation of a PTT-based conjugate fiber excellent in dyeing uniformity and ease of dyeing and suited to high speed false twisting, and a method of stably producing the fiber by a direct spin-draw process is strongly desired.